Environmental Issues Global Warming In 1975

Source: After Houghton et al. (1990)

example, the nature and rate of industrial development, the extent to which the earth's forests continue to be destroyed and the success of programmes aimed at reducing the output of CO2. Most studies have based their predictions on several scenarios, one of which is commonly a direct projection of the status quo—the IPCC 'business-as-usual' scenario, for example—with others based on either increase or decreases in CO2 and combinations of other gases (Bolin et al. 1986, Houghton et al. 1990). The net result is that future greenhouse gas levels are usually presented as ranges of possibilities rather than discrete values (see Figure 7.7).

Since CO2 is a radiative forcing agent known to warm the atmosphere, the rising CO2 values can be translated into temperature increases. It has been estimated, for example, that a 0.3-0.6°C increase in the earth's surface temperature has taken place since 1900, at a rate broadly consistent with that expected from the rising levels of greenhouse gases (Houghton et al. 1990). Schneider (1987) claims that the earth is 0.5° warmer in the 1980s than it was in the 1880s. The change has not been even, however. The main increase took place between 1910 and 1940, and again after 1975 (Gadd 1992). Between 1940 and 1975, despite rising greenhouse levels, global temperatures declined, particularly in the northern hemisphere. In addition, analysis of the records suggests that the relatively rapid warming prior to 1940 was probably of natural origin (Folland et al. 1990). Such changes are well within the range of normal natural variations in global temperatures (Crane and Liss 1985), but Hansen and Lebedeff (1988) have calculated that the warming between the 1960s and 1980s was more rapid than that between the 1880s and 1940s, which suggests that the greenhouse warming may be beginning to emerge from the general background 'noise'. Hansen, of the Goddard Institute of Space Studies, claimed subsequently that the global greenhouse signal is sufficiently strong that a cause-and-effect relationship between the CO2 increase and global warming can be inferred (Climate

Institute 1988a). The controversy continues, however. Kheshgi and White (1993) concluded that it will not be possible to separate a greenhouse warming signal from the overall noise until more is known about the dimensions and causes of natural climate variability. Wigley and Barnett (1990), in their contribution to the IPCC Scientific Assessment, took the middle ground. They noted that there is as yet no evidence of an enhanced greenhouse effect in the observational record, but cautioned that this may be in part a function of the uncertainties and inadequacies in current investigative techniques. In short, although greenhouse-gas-induced warming may not have been detected, it does not follow that it does not exist.

Estimates of global warming are commonly obtained by employing atmospheric modelling techniques based on computerized General Circulation Models (GCMs) (see Table 2.4). To examine the impact of an enhanced greenhouse effect on temperature, for example, the CO2 component in the model is increased to a specific level. The computer program is allowed to run until equilibrium is established among the various climatic elements included in the model and the new temperatures have been reached (see Figure 7.8). This approach had produced a general consensus by the mid-1980s, that a doubling of CO2 levels would cause an average warming of 1.3-4°C (Manabe and Wetherald 1975; Cess and Potter 1984; Dickinson 1986; Bolin et al. 1986). The IPCC assessment produced values of 1.5-4.5°C, with a best estimate of 2.5°C. These results compare with the estimate of 4-6°C made by Arrhenius at the beginning of the century (Kellogg 1987). Smaller increases have been calculated by Newell and Dopplick (1979) who estimated that the temperatures above the tropical oceans would increase by only 0.03°C on average and by Idso (1980) who estimated a global increase of 0.26°C. These lower values are generally considered to be unrepresentative by most scientists investigating the problem, however (Cess and Potter 1984; Webster 1984).

Figure 7.8 Change in global surface temperature following a doubling of CO . (a) December, January and February, (b) June, July and August 2

Figure 7.8 Change in global surface temperature following a doubling of CO . (a) December, January and February, (b) June, July and August 2

Source: Compiled from data in Houghton et al. (1990)

Although the estimated temperature increases are not particularly impressive—mainly because they are average global values—evidence from past world temperature changes indicates that they are of a magnitude which could lead to significant changes in climate and climate-related activities. During the Climatic Optimum—the warm epoch following the last Ice Age—some 5,000 to 7,000 years ago, temperatures in North America and Europe were only 2-3°C higher than the present average, but they produced major environmental changes (Lamb 1977). Evidence from that time period, and from another warm spell in the early Middle Ages, 800 to 1,000

Figure 7.9 Change in global soil moisture levels following a doubling of CO . (a) December, January and February, (b) June, July and August 2

Figure 7.9 Change in global soil moisture levels following a doubling of CO . (a) December, January and February, (b) June, July and August 2

Source: Compiled from data in Houghton et al. (1990)

years ago, also suggests that the greatest impact of any change will be felt in mid to high latitudes in the northern hemisphere.

The models used to investigate global change are usually considered to provide less accurate results at the regional level, but they do support these past geographical trends. Following a doubling of CO2, the Canadian Climate Centre GCM indicates a warming of almost 5°C for the southern part of the country, summer and winter. In the north, the seasonal differences would be considerable, with a mean temperature increase of 8-12°C in winter, but less than 1°C in summer (Hengeveld 1991). Similar values are predicted for northern Russia, but in north-western Europe an annual increase of only 2-4°C is considered more likely (Mitchell et al. 1990). However, in the latter region, climate is strongly influenced by the north Atlantic oceanic circulation, and since reliable ocean/atmosphere models remain in the development stage, the accurate prediction of temperature change for such areas is difficult.

The estimated temperature increases at lower latitudes are generally less. According to predictions from China, temperatures will rise by 2-3°C in that country with increases exceeding 3°C only in the western interior (NCGCC 1990). In southern Europe, the Sahel, south-east Asia and Australia—all areas which received particular attention in the IPCC assessment—the predicted increases generally fall between 1-2°C, with only southern Europe expecting an increase of 3°C in the summer months (Mitchell et al. 1990).

Because the various elements in the atmospheric environment are closely interrelated, it is only to be expected that if temperature changes, changes in other elements will occur also. Moisture patterns are likely to be altered, for example (see Figure 7.9). In the mid-continental grasslands of North America, precipitation totals may be reduced by 10 to 20 per cent, while summer soil moisture levels may fall by as much as 50 per cent (Manabe and Wetherald 1986). IPPC estimates are slightly less, with summer precipitation declining by 5-10 per cent and soil moisture by 15-20 per cent, but confidence in these values is low (Mitchell et al. 1990). In northern and north-western China— where an average temperature increase of 2°C is expected—evaporation rates would increase by about 20 per cent, compounding existing problems of aridity in these areas (NCGCC 1990). Precipitation would be less frequent over most of Europe in summer and autumn, and throughout the year in the south (Wilson and Mitchell 1987), causing soil moisture levels to decline by as much as 25 per cent (Mitchell et al. 1990). According to some projections, more rainfall is possible in parts of Africa and southeast Asia (Wigley et al. 1986; Kellogg 1987). The latter would benefit from increases of up to 10 per cent in soil moisture during the summer months, but in parts of Africa such as the Sahel— where temperatures are expected to rise no more than 2°C—winter rainfall would decline by 510 per cent and summer precipitation increases of as much as 5 per cent would be insufficient to prevent a decline in soil moisture (Mitchell et al. 1990). Changes such as these in moisture regimes coupled with changes in the length and intensity of the growing season, would disrupt existing vegetation patterns, and require major alterations in agricultural activities in many areas.

The reality of the situation may only become apparent when the changes have occurred, for there are many variables in the predictions. The human factors, as always, are particularly unpredictable. Technology, politics, socioeconomic conditions and even demography can contribute to changes in the concentration of CO2, yet the nature and magnitude of the variations in these elements is almost impossible to predict.

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